Metal particles having an average particle diameter of 1 to 100 nm have been used for an integrity test conducted on a virus removal membrane formed of regenerated cellulose as viral substitute particles. There is a good correlation between removal of viruses and removal of gold particles used as substitute particles (see Patent Document 1). A metal or metal compound colloid solution that contains a water-soluble polymer dispersant having a nitrogen-containing group and is used for an integrity test conducted on a virus removal membrane formed of a synthetic polymer such as hydrophilized polyvinylidene fluoride and the like has been proposed (see Patent Document 2). A gold colloid solution that contains a nonionic surfactant has been proposed as a stable colloid solution, although the solution is not used for an integrity test conducted on a virus removal membrane (see Patent Document 3).
It is preferable to have a step of washing a membrane that has been used as a virus removal membrane in a practical integrity test to thereby reduce the amount of residue in the membrane as much as possible. A method of washing a synthetic membrane after use comprising using citric acid has been proposed (see Patent Documents 2 and 4). However, when using the metal or metal compound colloid solution disclosed in Patent Document 2, the integrity of a virus removal membrane that comprises a hydrophilized synthetic polymer and has been subjected to protein filtration and washing cannot necessarily be accurately tested.
Specifically, proteins and the like that remain after washing the virus removal membrane that comprises a hydrophilized synthetic polymer interact with the colloid solution disclosed in Patent Document 2 (i.e., there has been arisen a problem that proteins and the like that remain in the virus removal membrane adhere to the colloid).    [Patent Document 1] Japanese Patent No. 3328857    [Patent Document 2] WO 2005/007328    [Patent Document 3] Japanese Patent No. 2902954    [Patent Document 4] JP-A-2006-55784